1. Field of the Invention
The present invention relates to an image forming apparatus wherein an image is formed by electrostatically causing developer to a recording medium.
2. Related Background Art
In the past, there have been proposed various apparatuses for forming an image in response to image formation, among which, an image forming apparatus in which conductive magnetic toner is used and an image is formed by electrostatically adhering the toner to a recording medium is already known.
For example, as shown in FIG. 14, in such a conventional image forming apparatus, a magnet roller 53a disposed in a developing device 52 is rotated in a direction b with respect to an endless recording medium 51 moved in a direction a by means of a drive feed roller 50a and a driven feed roller 50b, so that conductive magnetic toner 55 is fed in a direction c onto recording electrodes 54 densely formed on a sleeve 53b made of non-magnetic material. And, by applying a signal voltage between the recording electrodes 54 and a conductive layer 51a of the recording medium in response to image information from a record controlling portion 56, an image is formed on the recording medium by selectively and electrostatically causing the toner to adhere to an insulation layer 51b of the recording medium. For example, when the signal voltage of +40 V is applied from the toner controlling portion 56, the toner 55 adheres to the recording medium 51, whereas, when the signal voltage is 0 V (i.e., the voltage is not applied), the toner does not adhere to the recording medium. By repeating the application and non-application of the toner, the image is formed on the recording medium.
After the image formed on the recording medium is displayed at an image display area 57, the toner 55 adhering to the recording medium 51 is removed from the recording medium by means of a cleaning member 58 made of conductive carbon fibers, conductive resin or conductive rubber, with the result that the toner is dropped into the developing device 52 to be re-used in a next image forming process.
FIG. 15 is an explanatory view for explaining and showing an image forming portion in an enlarged scale. The recording electrode 54 is constituted by a flexible wiring member and is fixedly mounted on the surface of the sleeve 53b by means of a two sided adhesive tape and the like. A conductor exposed portion 54a constituting an electrode portion is formed on an end of the recording electrode 54 near the recording medium 51, and a through opening 54b is formed in the recording electrode for passing the toner 55. The dimension of the through opening 54b is so selected that the amount of toner left from between the recording medium 51 and the conductor exposed portion 54a becomes less than an amount of toner entering through the through opening 54b. Consequently, as shown in FIG. 15, the toner is accumulated ahead of the conductor exposed portion 54a as the toner 55 is moved in the direction c. The reason for accumulating the toner is to obtain an image having a desired density or more.
Now, the explanation will be continued. By dividing the surface of the sleeve 53b into a portion A (between the through opening 54b and the conductor exposed portion 54a, the center angle of sleeve 53b is .theta.) and a portion B (between the conductor exposed portion 54a and the through opening 54b, the center angle of the sleeve 53b is (2.pi.-.theta.)). Incidentally, W denotes the total weight of the toner disposed on the sleeve; V denotes the toner feeding speed; A.sub.1 denotes the cross-sectional area between the recording medium and the conductor exposed portion; A.sub.2 denotes the area of the through opening; .rho. denotes the toner density; and D denotes the diameter of the sleeve.
With adequate separation between the recording medium 51 and the sleeve 53b, the toner 55 on the surface of the sleeve 53b is moved in a steady state by rotating the magnet roller 53a.
Then, the recording medium 51 approaches the sleeve 53b to set a predetermined gap between the recording medium 51 and the conductor exposed portion 54a. After a time t elapses, the toner amounts W.sub.A, W.sub.B disposed on the portions A and B, respectively, will be as follows: EQU At the portion A, EQU W.sub.A =W.times..theta./2.pi.+.rho.(A.sub.2 -A.sub.1)Vt; EQU At the portion B, EQU W.sub.B =W.times.(2.pi.-.theta.)/2.pi.-.rho.(A.sub.2 -A.sub.1)Vt.
Now, since the toner 55 is circulated on the sleeve 53b, when the required for effecting one revolution of the sleeve 53b is t.sub.1, the following equation is obtained: EQU t.sub.1 =.pi.D/V.
And, after the time t.sub.1 is elapsed, the toner amounts W.sub.At, W.sub.Bt will be as follows: EQU W.sub.At =W.times..theta./2.pi.+.rho.(A.sub.2 -A.sub.1).pi.D; and EQU W.sub.Bt =W.times.(2.pi.-.theta.)/2.pi.-.rho.(A.sub.2 -A.sub.1)
Here, A.sub.2 &gt;A.sub.1. Thus, the toner amounts will be in the steady state.
FIG. 16 shows, in an enlarged scale, that the toner 55 accumulated in the image forming portion is in a steady state.
Now, when the starting point and the terminal point of an image effective area along the toner feeding direction are E and F at the side of the recording medium 51, respectively, and are G and H at the side of the conductor exposed portion 54a, respectively, the force exerted on the toner in the image effective area EFGH will be considered.
When the configuration of the image effective area EFGH is a quadrilateral EFGH as shown in FIG. 17, and the angle between a line segment EF and a line segment GH is .theta..sub.1 the quadrilateral EFGH will be a portion of a wedge directed toward the toner feeding direction. When a force P acts in a direction perpendicular to the line segment EG, the toner 55 in the quadrilateral EFGH will be subjected to forces P/2 (sin .theta..sub.1 /2) directed toward a direction perpendicular to the line segments EF, GH, respectively.
Since the toner 55 is accumulated ahead of the conductor exposed portion 54a and the weight thereof becomes W.sub.At after the time t.sub.1 has elapsed as mentioned above, the force P resembles a force generated when an object having the weight of W.sub.At strikes a wall at a speed of V (i.e., P.varies.W.sub.At V). Thus, when the toner 55 in the image effective area EFGH is accumulated at the conductor exposed portion 54a, the toner is subjected to a compression force of P/2 (sin .theta..sub.1 /2).
On the other hand, as the toner is compressed, the toner resistance tends to be reduced, as shown in FIG. 18. That is to say, when the toner in the image effective area EFGH accumulates at the conductor exposed portion 54a, the toner resistance R.sub.1 is reduced more than that when the toner is not accumulated, with the result that the electrostatic attraction force F.sub.E is increased. By decreasing the toner resistance R.sub.1 to be lower than a predetermined value R.sub.M, the electrostatic attraction force F.sub.E acting on the toner chain becomes greater than the magnetic force F.sub.M of the magnet roller 53a tending to hold the toner chain at the side of the sleeve 53b, thereby increasing the amount of toner 55 attracted to the recording medium 51.
However, with the above-mentioned arrangement, since the toner is accumulated to reduce the toner resistance R.sub.1 lower than the predetermined value R.sub.M, the toner speed V must be set above a predetermined value V.sub.M and the gap area A.sub.2 must be set below a predetermined value A.sub.2M. The reason for this is that, when f is a function representative of the toner resistance, the following relation is established: ##EQU1##
Accordingly, the driving force of the motor required for driving the magnet roller 53a to feed the toner must be increased to be more than a predetermined level, thus increasing the power consumption and making the apparatus large-sized.
Further, since the dimension of the gap between the recording medium 51 and the conductor exposed portion 54a has, of course, an upper limit, the constructural elements such as the drive feed roller 50a, sleeve 53b and the like must be manufactured with high accuracy, thus increasing the manufacturing cost. In addition, when the longitudinal dimension of the apparatus is increased, since the manufacturing accuracy of the elements such as the drive feed roller 50a, sleeve 53b and the like often decreases, the gap between the recording medium 51 and the conductor exposed portion 54a must be set to have a greater value. In this case, however, since the resistance R.sub.1 of the toner chain is increased in proportion to the increase in the gap value, it is necessary to devise for obtaining the relation R.sub.1 .ltoreq.R.sub.M.
In order to obtain such a relation conventionally, the toner feeding speed V may be increased, or the vertex angle .theta..sub.1 of the wedge having the quadrilateral shape EFGH may be decreased by increasing outer diameters of the drive feed roller 50a and the sleeve 53b to increase the toner compressing force. However, in the former case, the driving force of the motor for driving the magnet roller 53a must be increased, with the result that the motor becomes large-sized, thus making the apparatus itself bulky. On the other hand, in the latter case, i.e., when the outer diameters of the drive feed roller 50a and the sleeve 53b are increased, the recording condition is improved at the conductor exposed portion 54a. However, it is difficult to remove the toner chain from the sleeve 53b immediately after the recording operation, with the result that the area sweeping the toner 55 adhered to the recording medium 51 is increased, thus creating an uneven image.